Many radiotelephones employ retractable antennas, i.e., antennas which are extendable and retractable out of the radiotelephone housing. The retractable antennas are electrically connected to a signal processing circuit positioned on an internally disposed printed circuit board. Unfortunately, the close proximity of the retracted antenna to certain electronic components in the radiotelephone housing can cause a variety of operational problems. For example, errant noises or radiated energy can enter the receiver and degrade the performance of the radiotelephone. Further, the close proximity of a user's hand to the retracted antenna can affect the radiation pattern of the radiotelephone and cause undesirable de-tuning effects.
In order to optimally operate, the signal processing circuit and the antenna should be interconnected such that their respective impedances are substantially "matched". However, a retractable antenna by its very nature has dynamic components, i.e., components which move or translate with respect to the housing and the printed circuit board. As such, a retractable antenna does not generally have a single impedance value which complicates the matching system. Instead, the retractable antenna typically generates largely different impedance values when in an extended versus a retracted position. Therefore, it is preferred that the impedance matching system alter the antenna's impedance to properly match the terminal's impedance both when the antenna is retracted and extended. Thus, as is well known to those of skill in the art, radiotelephones with retractable antennas typically include matching circuits, one associated with the extended position and one with the retracted position. In the extended position, the antenna typically operates with a half-wave (.lambda./2) load. In this situation, the associated impedance may rise as high as 600 Ohms. In contrast, in the retracted position, the antenna rod generally operates with a quarter-wave (.lambda./4) load with an impedance typically near 50 Ohms. Therefore, when the antenna is in the extended position an L-C matching circuit may be needed or desired to match out the additional impedance.
The physical configuration of the matching network is further complicated by the miniaturization of the radiotelephone and the internally disposed printed circuit board. Many of the more popular handheld telephones are undergoing miniaturization. Indeed, many of the contemporary models are only 11-12 centimeters in length. Because the printed circuit board is disposed inside the radiotelephone, its size is also shrinking, corresponding to the miniaturization of the portable radiotelephone. Unfortunately, as the printed circuit board decreases in size, the amount of space which is available to support desired operational and performance parameters as well as to separate electronic components of the radiotelephone is generally correspondingly reduced. Therefore, it is desirable to utilize efficiently and effectively the limited space in the radiotelephone and on the printed circuit board.
In the past, a variety of shielding devices for electronic components have been described. Many of these shields include a base wall fastened to an electronic component and a lid which attaches to the base wall. For example, U.S. Pat. No. 5,354,951 to Lange, Sr. et al. illustrates a multiple piece-shielding device. Unfortunately, this type of shield may not protect the retracted antenna from the closely located and susceptible electronic components. Further, this type of device which includes multiple pieces, can leak, which in turn can allow energy, noise, or RF radiation to enter and overload the receiver. Additionally this type of shielding fails to address the undesirable radiation pattern described above.